I tested a Chinese piezo driver on an exponential horn having a mouth ~3.5" by 13.5" with a throat to mouth path of about 5.5".

the graphs I made on and ~20 degrees off axis show the low end to go to about 2.2Khz then dropping off like a rock below (maybe 8th order? - probably a combo of piezo mechanical resonance and horn cutoff) -a mid power dip around 4Khz then a peak about 12Khz perhaps 9dB followed by a similar amplitude dip - after that the response goes out nicely to 20Khz wihtout much rolloff - the upper peak & dip - maybe result of phase plug?

anyhow - do all piezo drivers basically follow this type of response on this type horn? - just curious how to improve these things (I don't think hte heat-sealed driver can be easily disassembled)- the sound is "good' - would think "better" if the piezo driver was more "sophisticated" - but - I think this import driver can sometimes be sourced for $1 - hard to beat at that price!

Freddy